'08 Corolla Intermittent P0171

Hey all! Could use some fresh eyes on this Corolla that's be making me look stupid. I've included a lot of repair history and test data, so please read to the end before commenting.

This Corolla likes to set a P0171 according to a schedule that I have yet to get my head around. :silly: It came in several visits ago with rodent damage to the EVAP system. Got a purge valve, a vapor hose by the tank, and has been returning ever since. Other repairs have been done for other unrelated reasons, but the MIL and the P0171 reoccur.

It isn't running lean now, and almost never does any time we test drive the vehicle. I've attached scan data from a recent test drive:



These files are viewable for free with ShopStream Connect:

www.snapon.com/EN/US/Diagnostics/SSC/SSC-Download-Page

The trims tend towards lean, to be sure - +10-12%, depending on the conditions. But not enough to set a P0171 by themselves. No vacuum leaks have been found. Intake boot is new. Fuel quality shows no excessive ethanol. Low fuel delivery is not suspected due to the upstream A/F sensor readings during WOT. No misfires have ever been counted in OEM data or Mode $06.

So when did it set the P0171? I've attached the Freeze Frame data from the last two visits:



The common denominators seem to be: Cold startup, 5-7 minutes into a drive, 150-170 degrees coolant temperature, 45-50ish% lean correction. One was while cruising, the other was near idle.

What's caught my eye so far is the downstream O2 sensor. I distilled the Freeze Frame data down into some relevant PIDs and got them side by side:



In both cases, the downstream O2 is showing flat line lean near .12V, but the upstream A/F sensor is indicating slightly rich. Is this one of those cases where the downstream sensor has an abnormal amount of weight in the fuel control strategy? :huh:

Graphing data during a test drive shows somewhat erratic behavior from the downstream sensor:



It's also worth noting that the 'Total FT%' PID starts to show activity around 150 degrees of coolant temperature. That might track with what I'm seeing in the Freeze Frame? Or it might be a coincidence.

So watch it during a cold startup, right? I have. Didn't catch it acting up. :silly: There seems to be a specific set of conditions under which this thing isn't happy. I vaguely suspect something about the owners driving habits are contributing to this. Not that their habits are wrong, necessarily, just better at exposing the issue.

Anyway, I'm a bit brain fried on this one. :silly: I want to change parts, but can't justify it with anything other than vague suspicions. As always, any thoughts/ideas/suggestions you have are appreciated.

Comparing the idle freeze frame against the good test drive MAF g/s looks low in the freeze frame. I know you said no vacuum leaks found but has this thing been smoked after a cold soak? One thing I noticed in the freeze frames is both set after a cold soak.

Tyler wrote:

What's caught my eye so far is the downstream O2 sensor. I distilled the Freeze Frame data down into some relevant PIDs and got them side by side:

In both cases, the downstream O2 is showing flat line lean near .12V

Graphing data during a test drive shows somewhat erratic behavior from the downstream sensor:

Yeah maybe the heater is out?? But I don't think the downstream O2 is THE problem. The rear O2 is also lean on the known good I lifted from the test drive.

I’ve seen them with the intake gaskets leak cold. Smoke test the intake cold.

Thinking back to the previous repair, is it running lean during purge?
If the evap monitor has passed then probably not the case, but I can picture a chewed through hose somewhere between the engine and cannister not really effecting anything until the purge opens, then vacuum leak city.
Just trying to think outside the box...

Comparing the idle freeze frame against the good test drive MAF g/s looks low in the freeze frame. I know you said no vacuum leaks found but has this thing been smoked after a cold soak? One thing I noticed in the freeze frames is both set after a cold soak.

I’ve seen them with the intake gaskets leak cold. Smoke test the intake cold.

I have not smoked it cold - the shop smoke machine is out for repair, and the Amazon one I tried was a POS. :lol: I didn't save it for whatever reason, but I did observe fuel trims during a start AFTER a cold soak. Well within 10% of zero.

Yeah maybe the heater is out?? But I don't think the downstream O2 is THE problem. The rear O2 is also lean on the known good I lifted from the test drive.

That's one of my concerns. I feel confident the rear O2 is janky, but I'm not convinced it's the cause of the P0171.

I may try disconnecting the downstream sensor and going for a test drive to recheck trim behavior.

Thinking back to the previous repair, is it running lean during purge?
If the evap monitor has passed then probably not the case, but I can picture a chewed through hose somewhere between the engine and cannister not really effecting anything until the purge opens, then vacuum leak city.

Interesting you mention that. I've actually warrantied out the aftermarket purge valve with an OE one due to sticking open intermittently. Caught it creating a massive vacuum leak on two occasions. :ohmy: After an OE purge went on, there have been no other EVAP issues.

What sucks is that the Freeze Frame data shows commands for the vent and purge, but not the fuel tank pressure.

Just trying to think outside the box...

I like outside the box! That's why I bring the fun ones here.

I have not smoked it cold - the shop smoke machine is out for repair, and the Amazon one I tried was a POS.

No problem, Tyler. Check out this awesome video .

Chad wrote:

No problem, Tyler. Check out this awesome video .

I'm never trusting a link from you again. :silly:

I did get the Corolla to act up today. I've attached the scan data file:





Starting cold-ish, there's no trim issues. You can see where the trims go excessively positive after frame 1200. This was after slowing down from highway speeds. Downstream O2 is hanging out at .12V like usual.

After watching the trims for a moment at a stop, I snapped the throttle briefly, just trying to get the downstream O2 to swtich:



Once the downstream sensor switched rich, the trims started working themselves out.

Really wanna throw a downstream sensor at it. :silly: Anything I'm missing?

Tyler, did you test for exhaust leaks?

Question about your front o2 in your frames you have marked. Why is the front o2 sitting at 3.14 volts but the trims are at 28%? I thought anything below 3.3 volts was rich and anything abbove 3.3 lean. But the rear o2 is dead lean. Almost seems like the front o2 is not working. Because if the rear was dead lean I’d expect a high voltage on the front o2.

Pic 2 the front o2 is 2.7 volts and rear o2 is full rich

Paul P. wrote:

Tyler, did you test for exhaust leaks?

Sure did, none found.

Pic 2 the front o2 is 2.7 volts and rear o2 is full rich

I think that's because there was a throttle snap right at that moment? I tried to zoom in on the throttle position PID in that shot, but it still doesn't show up very well.

Question about your front o2 in your frames you have marked. Why is the front o2 sitting at 3.14 volts but the trims are at 28%? I thought anything below 3.3 volts was rich and anything abbove 3.3 lean. But the rear o2 is dead lean. Almost seems like the front o2 is not working. Because if the rear was dead lean I’d expect a high voltage on the front o2.

I've been questioning which sensor is lying the entire time. :blush: The way I understand it, if the upstream A/F sensor decided to stick slightly rich, then any fuel the PCM adds in response would be excessive. That should show up as a rich downstream... Right? :huh:

I got the OK to throw a downstream O2 at it this afternoon. I saved the capture but didn't have time to post it here. There was no fuel trim issues afterwards. But that doesn't really mean much given how intermittent the problem is. :silly:

Wonder if there were any reflashes available for any type of symptoms you were getting

Cheryl wrote:

Wonder if there were any reflashes available for any type of symptoms you were getting

That's a fair question. :huh: I didn't see a reflash mentioned in a TSB (which they usually are on Toyota), but I can check anyway.

I ended up putting a Denso downstream sensor in this Tuesday afternoon and went for a drive:



It didn't act up that time. But most of the time, it didn't act up anyway. :silly: It took 300 miles to set this fault last time around, it's gonna be a bit before we know if the O2 was the fix or not.

As always, thanks to everyone who took the time to respond. I'm sure I'll be hearing about it if this one codes again.

If the rodent chewed the heater circuit wire then the heater would be open circuit. But, if this was the case you would get a MIL light with DTC P0135 after two qualifying drive cycles.